Apparatus and method for optical characterization of a sample over a broadband of wavelengths while minimizing polarization changes

ABSTRACT

An apparatus and method for optically characterizing the reflection and transmission properties of a sample with a beam of light having a small diameter on a surface of the sample over a broadband of wavelengths, from 190 nm to 1100 nm. Reflective optical components, including off-axis parabolic mirrors with a collimated incident or reflected broadband beam of light, minimize non-chromatic aberration. Angles of incidence and reflection from optical components and the sample are kept substantially near normal to the optical components and the sample to minimize changes in the polarization of the beam of light. The apparatus and method further disclose an optical light path that can be focused by adjusting the position of an off-axis parabolic mirror and a planar mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is co-filed with application “Apparatus andMethod for Optical Characterization of a Sample Over a Broadband ofWavelengths With a Small Spot Size” by Ray Hebert, Marc Aho and AbdulRahim Forouhi, which is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method foroptically characterizing the properties of a sample on reflection andtransmission of light over a broadband of wavelengths while minimizingpolarization changes.

BACKGROUND OF THE INVENTION

Advances in microelectronics necessitate components with ever smallercritical dimensions. Manufacturing such components requires the use ofshorter wavelengths of light in the lithography processes employed incomponent fabrication. This, in turn, has lead to a need to measure theoptical characteristics of samples such as, among other,photolithographic masks and fabricated components over a broad range orbroadband of wavelengths including the UV. Typically, in thesemeasurements the cross-sectional diameter of a beam of light focused onthe sample is large enough to spatially average the opticalcharacteristic being measured yet small enough to resolve spatialvariations across the sample. As the critical dimensions have decreasedso too has the required diameter of the beam of light on the sample. Itis now desirable to have a diameter of less than 100 micron.

As with many engineering problems, the design of an optical system tomeasure the optical characteristics of such a sample represents atradeoff. For example, when illuminating the beam of light with thebroadband of wavelengths from a light source onto a surface of thesample, it is desirable to have a small spot size but not a diffractionlimited spot. In addition, this should be accomplished in an opticallyefficient manner. There is, therefore, a tradeoff in this regard betweena need for optical components with a low f-number (for higher opticalefficiency) and a need for optical components with a high f-number (fora small spot size over a practical depth of field with minimalaberration and angles of incidence) and thus a small cone of rayscorresponding to the beam of light that is used in the optical system,i.e., the useful light. Similar design tradeoffs occur in the collectionand illumination on a detector of the beam of light reflected from thesample and the beam of light transmitted through the sample.

The need to operate over the broadband of wavelengths is a furtherdesign constraint for with many optical components because they aresubject to a variety of effects such as chromatic aberration andabsorption. For refractive optical components these effects becomepronounced as the wavelengths approach the UV. There exist opticalsystems based on refractive optical components in the prior art thatoperate over a broadband of wavelengths with a small diameter of thebeam of light on the sample. In these systems, attempts are made tocompensate for chromatic aberration and absorption effects. However,this adds expense and complexity to these optical systems.

Reflective optical components are a suitable solution to this technicalchallenge. A wide variety of components are available including mirrorswith non-spherical shape, such as an off-axis paraboloid shape,henceforth called an off-axis parabolic mirror. However, non-sphericalshaped mirrors can add expense to the optical system, especially whensuch mirrors are manufactured by diamond turning. Optical systemsincluding torroidal, spherical and elliptical mirrors are disclosed inthe prior art. For examples, see U.S. Pat. No. 5,910,842, U.S. Pat. No.6,583,877 and U.S. Pat. No. 6,128,085.

In addition, many prior art broadband optical systems combine refractiveand reflective optical components. However, such catadioptric systems donot avoid the complexity and expense needed to overcome the chromaticaberration and absorption issues associated with refractive opticalcomponents.

Furthermore, when different samples are characterized, the beam of lightin the optical system will need to be focused on the sample to correctfor effects such as varying surface topography. Such an adjustment isproblematic if the adjustment of the position of certain opticalcomponents in the optical system necessitates the adjustment of theposition of many other optical components, since this can easily lead tomisalignment. A preferred solution would allow the beam of light to befocused on the sample by adjusting a minimum number of components in theoptical system or a simple assembly of components. Furthermore, such apreferred solution would be a sufficiently compact and simple opticalsystem that a single light source could be used to opticallycharacterize the reflection and transmission properties of the sample.

Addition information in the optical characterization of the sample canbe obtained by selectively polarizing the beam of light illuminating thesample, analyzing the polarization of the beam of light reflected off ofor transmitted through the sample, or both. This poses yet anothertechnical challenge, since it is known that the polarization of the beamof light is changed on reflection from or transmission throughmaterials. It would be beneficial if such polarization changesassociated with the optical components and the sample substrate could beminimized.

There is a continued need, therefore, for a compact optical system foroptical characterization of a sample, which operates over a broadband ofwavelengths with a small diameter of the beam of light on the sample andwhich employs reflective optics with a minimum number of opticalcomponents such that advantageous components such as off-axis parabolicmirrors can be used. There is also a need for such an optical systemthat can be focused by adjusting the position of the minimum number ofoptical components or a simple assembly of components, and for anoptical system that minimizes changes in the polarization of the beam oflight.

OBJECTS AND ADVANTAGES

In view of the above, it is a primary object of the present invention toprovide an apparatus and method that enables optical characterization ofthe properties of a sample on reflection and transmission of a beam oflight over a broadband of wavelengths with a small spot size on thesurface of the sample while minimizing changes in the polarization ofthe light. More specifically, it is an object of the present inventionto provide a broadband apparatus with a small spot size on the surfaceof the sample, and a method of using this apparatus, for opticalcharacterization of the properties of the sample on reflection andtransmission of the beam of light through the use of optical light pathscomprising reflective optical components, including off-axis parabolicmirrors, which minimize changes in the polarization of the beam oflight. It is a further object of the present invention to provide anapparatus, and a method of using this apparatus, where the spot size onthe surface of the sample can be brought into focus without extensiveadjustment of the position of these optical light paths.

These and numerous other objects and advantages of the present inventionwill become apparent upon reading the following description.

SUMMARY

The objects and advantages of the present invention are secured by anapparatus and method for the optical characterization of the propertiesof a sample on reflection and transmission of a beam of light, with asmall spot size on the sample, over a broadband of wavelengths. Abroadband beam of light from a light source is fractionally magnifiedand illuminated onto a top surface of the sample. A portion of thebroadband beam of light is reflected from the top surface of the sample,a portion of the broadband beam of light is transmitted through thesample and a portion of the broadband beam of light is absorbed. Theportion of the broadband beam of light reflected from the top surface ofthe sample is redirected and illuminated onto a first detector. Theportion of the broadband beam of light transmitted through the sample isredirected from a bottom surface of the sample and illuminated onto asecond detector. These functions are accomplished using an illuminationoptical light path, a reflection optical light path and a transmissionoptical light path, each of which comprises reflective opticalcomponents, thereby eliminating chromatic aberrations from thesecomponents. Pairs of planar and off-axis parabolic mirrors are used toredirect and magnify the broadband beam of light. In a preferredembodiment, the planar and off-axis parabolic mirrors are coated with aUV-enhancing aluminum coating. The broadband beam of light in theillumination optical light path, the reflection optical light path andthe transmission optical light path is collimated between the pair ofparabolic mirrors in each optical light path. This configuration allowsfocusing of the broadband beam of light on the top surface of the sampleby adjusting a position of one of the pairs of planar and off-axisparabolic mirrors without requiring adjustment of the position of othercomponents in each of the optical light paths.

In a preferred embodiment, polarization changes in the beam of light inthe illumination optical light path, the reflection optical light pathand the transmission optical light path are minimized by ensuring thatangles of incidence and reflection of the broadband beam of light fromthe planar and off-axis parabolic mirrors in each of the light paths aresmall (near the normal to the mirrors) and that the angles in the beamof light illuminated on, reflected from or transmitted through thesample are near normal to the top surface or the bottom surface of thesample.

In another embodiment of this invention, an optical fiber is used toredirect the portion of the broadband beam of light transmitted throughthe sample to illuminate the second detector.

In another embodiment of this invention, a polarizing means isincorporated into at least one of the optical light paths to adjust thepolarization of the broadband band beam of light.

In another embodiment of this invention, the portion of the broadbandbeam of light reflected from the sample and the portion of the broadbandbeam of light transmitted through the sample are each redirected andilluminated onto a common detector.

A detailed description of the invention and the preferred andalternative embodiments is presented below in reference to the attacheddrawing figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram illustrating an apparatus according to theinvention.

FIG. 2 is a diagram illustrating another embodiment of an apparatusaccording to the invention.

FIG. 3 is a diagram illustrating a side view of one of the planarmirrors of the apparatus in FIG. 1 or FIG. 2.

FIG. 4 is a diagram illustrating a side view of one of the off-axisparabolic mirrors of the apparatus in FIG. 1 or FIG. 2.

FIG. 5 is a diagram illustrating the focusing of the broadband beam oflight onto the sample surface.

FIG. 6 is a diagram illustrating a cross-sectional view of the broadbandbeam of light.

FIG. 7 is a diagram illustrating the beam of light incident, reflectedand transmitted from a surface with the polarization of the lightperpendicular to the plane defined by the incident and reflected light.

FIG. 8 is a diagram illustrating the beam of light incident, reflectedand transmitted from a surface with the polarization of the light in theplane defined by the incident and reflected light.

FIG. 9 shows the calculated amplitude coefficient as a function of theangle of incidence for light incident in air onto an aluminum object.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A preferred embodiment of the invention is illustrated in FIG. 1. Anapparatus 100 according to the invention comprises a first optical lightpath 110, a second optical light path 160 and a third optical light path180. A light source 112 produces the broadband beam of light 114 between190 nm and 1100 nm (broadband beam 114 is identified by its extremalrays in FIG. 1). An arc source, such as a Hamamatsu L2-2000 seriesdeuterium lamp, is suitable for the UV portion of the spectrum, and atungsten lamp for the visible and IR portions of the spectrum. Acombination of the deuterium lamp and the tungsten lamp is suitable asthe light source 112. The broadband beam of light 114 is redirected onreflection off of a first planar mirror 116. A model 01-MGF-005/028planar mirror from Melles-Griot is suitable as the planar mirror 116.

Referring to FIG. 3, the broadband beam of light 114 has a maximum angleof incidence 120 relative to a normal 118 to the planar mirror 116 and amaximum angle of reflection 122 relative to the normal 118 of the planarmirror 116. By keeping the maximum angle of incidence 120 and themaximum angle of reflection 122 small, that is, negligible with respectto the normal 118, changes in the polarization of the broadband beam oflight 114 are minimized. The same technique is used for planar mirrors138, 164, 170, 190 and 196 in the apparatus. This technique is furtherdescribed in FIGS. 7-9.

Referring to FIG. 7, two regions, 300 and 302 are separated by a surface304. There is incident light 312, with an angle of incidence 326relative to a normal 306, reflected light 318, with an angle ofreflection 328 relative to the normal 306, and transmitted light 324,with an angle of transmission 330 relative to the normal 306. Theelectric field 308 in incident light 312, the electric field 314 inreflected light 318 and the electric field 320 in transmitted light 324are directed perpendicular to the plane defined by the incident light312 and the reflected light 318. The incident light 312 has a magneticfield 310, the reflected light 314 has a magnetic field 316 and thetransmitted light 324 has a magnetic field 322. Referring to FIG. 8, inan alternate polarization the electric field 332 in the incident light312, the electric field 336 in the reflected light 318 and the electricfield 340 in the transmitted light 324 are directed in the plane definedby the incident light 312 and the reflected light 318. The incidentlight 312 has a magnetic field 334, the reflected light 318 has amagnetic field 338 and the transmitted light 324 has a magnetic field342. For this geometry, any electric field polarization can be obtainedby the linear superposition of the electric field polarizations shown inFIGS. 7 and 8. Thus, it is sufficient to consider the impact ofreflection and transmission for these two polarizations.

Referring to FIG. 9, the calculated amplitude coefficients usingFresnel's equations for light, in a medium in region 300 with an indexof refraction of 1.0, reflected off of and transmitted through aluminumin region 302, with an index of refraction of 1.39, as a function of theangle of incidence 326 is shown. The four amplitude coefficients arer_(∥), r_(⊥), t_(∥) and t_(⊥), where r_(∥) and t_(∥) is the ratio of theparallel component of the electric field 336 in the reflected light 318and the parallel component of the electric field 340 in the transmittedlight 324 relative to the parallel component of the electric field 332in the incident light 312, and r_(⊥) and t_(⊥) is the ratio of theperpendicular component of the electric field 314 in the reflected light318 and the perpendicular component of the electric field 320 in thetransmitted light 324 relative to the perpendicular component of theelectric field 308 in the incident light 312. As shown in FIG. 9, theamplitude coefficients, and thus the polarization of the light, aresubstantially unchanged for angle of incidence 326 (and, by symmetry,for angle of reflection 328) substantially less than 30 degrees from thenormal 306 to the surface 304. The result that the polarization issubstantially unchanged for angle incidence 326 (angle of reflection328) negligible with respect to the normal 306 to the surface 304 isunchanged for values of the index of refraction in region 302substantially the same as that used in the calculation shown in FIG. 9.

Referring back to FIG. 1, the first planar mirror 116 is positionedrelative to a first off-axis parabolic mirror 126 such that thebroadband beam of light 114 is collimated on reflection from the firstoff-axis parabolic mirror 126. A suitable off-axis parabolic mirror canbe custom manufactured by Edmond Industrial Optics using diamondturning. A commercially available example of such an off-axis parabolicmirror is model H47-085 from Edmond Industrial Optics. Referring to FIG.4, the broadband beam of light 114 has a maximum angle of incidence 130and a maximum angle of reflection 132 relative to a normal 128 to theoff-axis parabolic mirror 126. By keeping the maximum angle of incidence130 and the maximum angle of reflection 132 small, that is, negligiblewith respect to the normal 128, changes to the polarization of thebroadband beam of light 114 are minimized. The same technique is usedfor off-axis parabolic mirrors 140, 162, 168, 188 and 194 in theapparatus.

Referring back to FIG. 1, the broadband beam of light 114 is redirectedon reflection off of a second planar mirror 138. The broadband beam oflight 114 incident and reflected off of the second planar mirror 138 iscollimated. The second planar mirror 138 is positioned relative to asecond off-axis parabolic mirror 140 such that the broadband beam oflight 114 illuminates and is brought into focus on a sample 144.

Referring to FIG. 5, the broadband beam of light 114 is collimated whenincident on the second off-axis parabolic mirror 140. This ensures thatthe broadband beam of light 114 will come to focus at a distance 142from the second off-axis parabolic mirror 140. There is a knownrelationship between the distance 142 and focal length along axis of thesecond off-axis parabolic mirror 140. A person of skill in the art willbe able to determine the focal length from the curvature of the secondoff-axis parabolic mirror 140. By adjusting the position of the secondoff-axis parabolic mirror 140 relative to the sample 144, the broadbandbeam of light 114 is brought into focus on a top surface 146 of thesample 144. It is important, however, that the position of the secondplanar mirror 138 be adjusted such that the second planar mirror 138maintains the same position relative to the second off-axis parabolicmirror 140. In this way, the collimated light reflected off of thesecond planar mirror 138 remains parallel to the axis (not shown) of thesecond off-axis parabolic mirror 140. Since the broadband beam of light114 incident on the second planar mirror 138 is collimated, thisadjustment of the position of the second planar mirror 138 and thesecond off-axis parabolic mirror 140 does not necessitate adjustment ofthe position of the other components in the first optical light path110.

Referring to FIG. 6, the broadband beam of light 114 has a cross-section210 with a diameter 220 defined as twice the distance from the center ofthe cross-section 210 where the light intensity is reduced by a factorof 1/e. The broadband beam of light 114 has a diameter 220 greater than500 microns at the light source 112 and a diameter 220 between 50microns and 80 microns on the top surface 146 of the sample 144. Thisreduction is proportional to the ratio of the focal lengths of off-axisparabolic mirror 140 and off-axis parabolic mirror 126.

Referring back to FIG. 5, the small diameter 220 of the broadband beamof light 114 illuminated on the top surface 146 of the sample 144corresponds to a small spread of angles in the cone of rays in thebroadband beam of light 114 incident on the sample 144. The broadbandbeam of light 114 incident on the top surface 146 of the sample 144 hasa minimum angle of incidence 152 and a maximum angle of incidence 154relative to a normal 150 to the top surface 146 of the sample 144. Bykeeping the maximum angle of incidence 154 small, that is, negligiblewith respect to the normal 150, changes to the polarization of thebroadband beam of light 114 are minimized.

Referring back to FIG. 1, the broadband beam of light 161 is reflectedfrom the top surface 146 of the sample 144 (broadband beam 161 isidentified by its extremal rays in FIG. 1). The broadband beam of light161 is redirected and magnified in the second optical light path 160.The broadband beam of light 161 is redirected on reflection off of afirst off-axis parabolic mirror 162 and then redirected on reflectionoff of a first planar mirror 164. In a manner similar to that used inadjusting the position of second planar mirror 138 and second off-axisparabolic mirror 140 in the first optical light path 110, the positionof the first off-axis parabolic mirror 162 and the first planar mirror164 relative to the top surface 146 of the sample 144 are adjusted suchthat the broadband beam of light 161 incident and reflected from thefirst planar mirror 164 is collimated. This ensures that the adjustmentof the position of the first off-axis parabolic mirror 162 and theadjustment of the position of the first planar mirror 164 does notnecessitate adjustment of the position of other components in the secondoptical light path 160. Referring back to FIG. 5, the small diameter 220of the broadband beam of light 114 on the top surface 146 of the sample144 corresponds to a small spread of angles in the cone of rays in thebroadband beam 161 of light with a minimum angle of reflection 156 and amaximum angle of reflection 158. By keeping the maximum angle ofreflection 158 small, that is, negligible with respect to the normal150, changes to the polarization of the broadband beam of light 161 areminimized.

Referring back to FIG. 1, the broadband beam of light 161 is redirectedon reflection off of a second of-axis parabolic mirror 168. Thebroadband beam of light 161 is redirected on reflection off of thesecond planar mirror 170 and illuminates a first detector 172. Theentrance aperture 171 of the first detector 172 is positioned at thefocal length of the second off-axis parabolic mirror 168. A person ofskill in the art will be able to determine the focal length from thecurvature of the second off-axis parabolic mirror 168.

Referring back to FIG. 5, after transmission through the sample 144 thebroadband beam of light 181 exits the sample through a bottom surface148 of the sample 144 (broadband beam is 181 is identified by itsextremal rays in FIG. 5). The cone of rays in the broadband beam oflight 181 transmitted through the sample 144 has minimum angle oftransmission 184 and maximum angle of transmission 186 relative to anormal 182 to the bottom surface 148 of the sample 144. By keeping themaximum angle of transmission 186 small, that is, negligible withrespect to the normal 182, changes to the polarization of the broadbandbeam of light 181 are minimized. Referring back to FIG. 1, the broadbandbeam of light 181 is redirected and magnified by the third optical lightpath 180. The broadband beam of light 181 is redirected on reflectionoff of a first off-axis parabolic mirror 188 and then redirected onreflection off of a first planar mirror 190. In a manner similar to thatused in adjusting the position of second planar mirror 138 and secondoff-axis parabolic mirror 140 in the first optical light path 110, theposition of the first off-axis parabolic mirror 188 and the first planarmirror 190 relative to the top surface 146 of the sample 144 areadjusted such that the broadband beam of light 181 incident andreflected from the first planar mirror 190 is collimated. This ensuresthat the adjustment of the position of the first off-axis parabolicmirror 188 and the adjustment of the position of the first planar mirror190 does not necessitate adjustment of the position of other componentsin the third optical light path 180. Since the broadband beam of light114, 161 and 181 is collimated substantially perpendicular to the sample144 over a portion of the first optical light path 110, the secondoptical light path 160 and the third optical light path 180, in anembodiment of this invention the adjustment of the second planar mirror138 and second off-axis parabolic mirror 140, the first off-axisparabolic mirror 162 and the first planar mirror 164, and the firstoff-axis parabolic mirror 188 and the first planar mirror 190 relativeto the top surface 146 of the sample 144 is accomplished with a group ofmechanically coupled elements. The broadband beam of light 181 isredirected on reflection off of a second of-axis parabolic mirror 194.The broadband beam of light 181 is redirected on reflection off of thesecond planar mirror 196 and illuminates a second detector 198. Theentrance aperture 197 of the second detector 198 is positioned at thefocal length of the second off-axis parabolic mirror 194. A person ofskill in the art will be able to determine the focal length from thecurvature of the second off-axis parabolic mirror 194.

FIG. 3 illustrates a side view of the first planar mirror 116 in thefirst optical light path 110. In a preferred embodiment of theinvention, the planar mirror 116 includes a UV-enhancing aluminumcoating 124. As an example, the model 01-MGF-005/028 planar mirror fromMelles-Griot has a UV-enhancing aluminum coating 124. In a preferredembodiment, such UV-enhancing aluminum coatings are used on the otherplanar mirrors 138, 164, 170, 190 and 196 in the first optical lightpath 110, the second optical light path 160 and the third optical lightpath 180.

FIG. 4 illustrates a side view of the first off-axis parabolic mirror126 in the first optical light path 110. In a preferred embodiment ofthe invention, the off-axis parabolic mirror 126 includes a UV-enhancingaluminum coating 134. Edmond Industrial Optics is a supplier of suchUV-enhanced aluminum coatings. In a preferred embodiment, suchUV-enhancing aluminum coatings are used on the other off-axis parabolicmirrors 140, 162, 168, 188 and 194 in the first optical light path 110,the second optical light path 160 and the third optical light path 180.

FIG. 2 illustrates alternate embodiments of the invention. The firstoptical light path 110 includes a polarizing means 136 for polarizingthe broadband beam of light 114 in one of two orthogonal directions. Asuitable device is a model PTH-SMP Glan Thompson-type calcite polarizermade by Harrick. The second optical light path 160 includes a polarizingmeans 166, such as a polarizing analyzer. Once again, the model PTH-SMPGlan Thompson-type calcite polarizer made by Harrick is suitable. Thethird optical light path 180 includes a polarizing means 192, such as apolarizing analyzer.

In another embodiment of this invention, the third optical light path180 also includes an optical fiber 199 for redirecting the broadbandbeam 181 from the third optical light path 180 to the second detector198.

In another embodiment of this invention, the broadband beam 181 from thethird optical light path 180 is redirected and illuminated onto thefirst detector 172 eliminating the need for the second detector 198.Additional optical components, such as a beam splitter, may be added asis known in the art to ensure that broadband beam 161 and broadband beam181 are coaxial when they illuminate the first detector 172. A choppermay also be added.

Referring back to FIG. 1, the first detector 172 and the second detector198 depend on the type of optical characterization to be performed onthe sample 144. For measurements of reflected or transmitted intensityas a function of wavelength, the first detector 172 and the seconddetector 198 with a monochromator, a diode array or a photomutipliertube is suitable. A monochromator with a 512-element diode array (ModelPDA-512) is available from Control Development. A mechanically scannedmonochromator is known in the art. A suitable photomultiplier is modelR928 from Hamamatsu. For a spectroscopic ellipsometer, a polarizationanalyzer, such as the model PTH-SMP Glan Thompson-type calcite polarizermade by Harrick, in addition to the monochromator, the diode array orthe photomultiplier tube is suitable. In one embodiment, thepolarization analyzer can be incorporated in the first detector 172 andthe second detector 198. The analysis techniques in U.S. Pat. No.4,905,170 to Forouhi et al. and U.S. patent application Ser. No.10/607,410 to Li et al., hereby incorporated by reference, can be usedto determine optical characteristics of the sample 144 from themeasurements.

By employing substantially reflective optical components and off-axisparabolic mirrors with collimated incident broadband beam of light 114,reflected broadband beam of light 161, and transmitted broadband beam oflight 181, the invention minimizes chromatic aberration in the firstlight path 110, the second light path 160 and the third light path 180.This enables the small diameter 220 of the broadband beam of light 114and 161 on the top surface 146 of the sample 144 as well as opticalcharacterization of reflection and transmission properties using thesingle light source 112. The diameter 220 of the broadband beam of light114 and 161 is small enough to resolve spatial variations in opticalcharacteristics on the top surface 146 of the sample 144 yet largeenough to spatially average the optical characteristics of the sample144. Artifacts associated with diamond-turned parabolic mirrors are nota concern in this invention since the diameter 220 of the broadband beamof light 114 and 161 on the top surface 146 and the diameter 220 of thebroadband beam of light 181 on the bottom surface 148 of the sample 144are not diffraction limited. The principle impact of such artifacts isscattering of the broadband beam of light 114, 161 and 181, which is nota concern in this invention since these scattered rays will not beilluminated onto the first detector 172 or the second detector 198.

Furthermore, by ensuring that the maximum angle of incidence 120 andreflection 122 for the planar mirrors 116, 138, 164, 170, 190 and 196are small, that is, negligible with respect to the normal 118, that themaximum angle of incidence 130 and reflection 132 for the off-axisparabolic mirrors 126, 140, 162, 168, 188 and 194 are small, that is,negligible with respect to the normal 128, that the maximum angle ofincidence 154 and the maximum angle of reflection 158 from the topsurface 146 are small, that is, negligible with respect to the normal150, and that the maximum angle of transmission 186 is small, that is,negligible with respect to the normal 182 to the bottom surface 148changes to the polarization of the broadband beam of light 114, 161 and181 are minimized.

The first, second and third optical light paths 110, 160 and 180 in thisinvention have been described with parabolic mirrors 126, 140, 162, 168,188 and 194. One skilled in the art will recognize that other mirrorshapes such as a toroidal mirror as well as those based on conicsections, such as elliptical, hyperbolic and spherical, are alsosuitable. In addition, another reflective surface may be substituted forthe planar mirrors 116, 138, 164, 170, 190 and 196.

In view of the above, it will be clear to one skilled in the art thatthe above embodiments may be altered in many ways without departing fromthe scope of the invention. Accordingly, the scope of the inventionshould be determined by the following claims and their legalequivalents.

1. An apparatus for characterizing optical properties of a sample,comprising: a) a light source for generating a broadband beam; b) atleast a first set of components defining a first light path, saidcomponents including at least a first component pair of a planar mirrorand a parabolic mirror with a first focal length and a second componentpair of a planar mirror and a parabolic mirror with a second focallength, wherein said broadband beam illuminates said planar mirror andsaid parabolic mirror in said first component pair and said planarmirror and said parabolic mirror in said second component pair at anglessubstantially near normal to said planar mirror and said parabolicmirror in said first component pair and said planar mirror and saidparabolic mirror in said second component pair; and c) an element ontowhich said broadband beam is illuminated, wherein said broadband beamilluminates said element at angles substantially near normal to saidelement.
 2. The apparatus of claim 1 wherein said planar mirror and saidparabolic mirror in said first component pair are positioned such thatsaid broadband beam exiting said first component pair is collimated. 3.The apparatus of claim 1 wherein said planar mirror and said parabolicmirror in said second component pair are positioned such that saidbroadband beam entering said second component pair is collimated.
 4. Theapparatus of claim 1 wherein said planar mirror and said parabolicmirror in said first component pair each has a UV-enhancing aluminumcoating.
 5. The apparatus of claim 1 wherein said planar mirror and saidparabolic mirror in said second component pair each has a UV-enhancingaluminum coating.
 6. The apparatus of claim 1 wherein said first focallength of said parabolic mirror in said first component pair isdifferent than said second focal length of said parabolic mirror in saidsecond component pair.
 7. The apparatus of claim 1 wherein said firstset of components further comprises a polarizing means.
 8. The apparatusof claim 7 wherein said polarizing means polarizes said broadband beamin one of two orthogonal directions.
 9. The apparatus of claim 7 whereinsaid polarizing means further comprises a rotatable polarizationanalyzer.
 10. The apparatus of claim 1 wherein said element is selectedfrom the group consisting of a sample and a first detector.
 11. Theapparatus of claim 10 further comprising a polarizing means in saidfirst detector.
 12. The apparatus of claim 11 wherein said polarizingmeans further comprises a rotatable polarization analyzer.
 13. Theapparatus of claim 10 wherein said first detector is a spectroscopicellipsometer.
 14. The apparatus of claim 1 wherein said broadband beamhas wavelengths lying in a range between 190 and 1100 nm.
 15. Theapparatus of claim 1 wherein said broadband beam has a diameter ofgreater than 500 μm at said light source and a diameter lying in a rangebetween 50 and 80 μm when illuminated onto a top surface of said sample.16. The apparatus of claim 1 further comprising a means of mechanicallydisplacing said second component pair while maintaining relativeposition of said parabolic mirror and said planar mirror such thatdistance from said parabolic mirror and a top surface of said sample issuch that said broadband beam is focused.
 17. The apparatus of claim 1further comprising a second set of components defining a second lightpath, wherein said element is a first detector.
 18. The apparatus ofclaim 1 further comprising a third set of components defining a thirdlight path.
 19. The apparatus of claim 18 wherein said first focallength of said parabolic mirror in said first component pair in saidthird light path and said second focal length of said parabolic mirrorin said second component pair in said third light path are differentthan said first focal length of said parabolic mirror in said firstcomponent pair in said first light path and said second focal length ofsaid parabolic mirror in said second component pair in said first lightpath.
 20. The apparatus of claim 18 wherein said element is a firstdetector.
 21. The apparatus of claim 18 wherein said element is a seconddetector.
 22. The apparatus of claim 21 further comprising a polarizingmeans in said second detector.
 23. The apparatus of claim 22 whereinsaid polarizing means further comprises a rotatable polarizationanalyzer.
 24. The apparatus of claim 21 wherein said second detector isa spectroscopic ellipsometer.
 25. The apparatus of claim 18 furthercomprising a fiber for redirecting said broadband beam.
 26. A method ofcharacterizing optical properties of a sample comprising the steps of:a) providing a sample to be characterized; b) generating light in abroadband beam; c) magnifying and illuminating said broadband beam ontoa top surface of said sample in a first set of reflective componentsdefining a first light path, wherein changes in polarization of saidbroadband beam are minimized by ensuring that said broadband beamilluminates said reflective components in said first light path and saidsample at angles substantially near normal to said reflective componentsand said sample; d) magnifying and illuminating said broadband beamreflected from said top surface of said sample to a first detector in asecond set of reflective components defining a second light path,wherein changes in polarization of said broadband beam are minimized byensuring that said broadband beam illuminates said reflective componentsin said second light path and said sample at angles substantially nearnormal to said reflective components and said sample; e) measuring anintensity of said broadband beam reflected from said top surface of saidsample with said first detector; and f) determining optical propertiesof said sample based on said intensity of said broadband beam reflectedfrom said top surface of said sample.
 27. The method of claim 26 furthercomprising the step of polarizing said broadband beam in said firstlight path in one of two orthogonal directions.
 28. The method of claim26 further comprising the step of focusing said broadband beamilluminating said top surface of said sample in said first light path.29. The method of claim 26 further comprising the step of focusing saidbroadband beam reflected from said top surface of said sample in saidsecond light path.
 30. The method of claim 26 further comprising thestep of adjusting polarization of said broadband beam reflected fromsaid top surface of said sample in said second light path.
 31. Themethod of claim 26 wherein said broadband beam has wavelengths lying ina range between 190 and 1100 nm.
 32. The method of claim 26 furthercomprising the steps: g) magnifying and illuminating said broadband beamfrom a bottom surface of said sample, after transmission of saidbroadband beam from said top surface of said sample through said sample,to a second detector in a third set of reflective components defining athird light path, wherein changes in polarization of said broadband beamare minimized by ensuring that said broadband beam illuminates saidreflective components in said third light path and said sample at anglessubstantially near normal to said reflective components and said sample;h) measuring an intensity of said broadband beam from said bottomsurface of said sample, after transmission of said broadband beam fromsaid top surface of said sample through said sample, with said seconddetector; and i) determining optical properties of said sample based onsaid intensity of said broadband beam from said bottom surface of saidsample, after transmission of said broadband beam from said top surfaceof said sample through said sample.
 33. The method of claim 26 furthercomprising the steps: g) magnifying and illuminating said broadband beamfrom a bottom surface of said sample, after transmission of saidbroadband beam from said top surface of said sample through said sample,to said first detector in a third set of reflective components defininga third light path, wherein changes in polarization of said broadbandbeam are minimized by ensuring that said broadband beam illuminates saidreflective components in said third light path and said sample at anglessubstantially near normal to said reflective components and said sample;h) measuring an intensity of said broadband beam from said bottomsurface of said sample, after transmission of said broadband beam fromsaid top surface of said sample through said sample, with said firstdetector; and i) determining optical properties of said sample based onsaid intensity of said broadband beam from said bottom surface of saidsample, after transmission of said broadband beam from said top surfaceof said sample through said sample.
 34. The method of claim 26 furthercomprising the step of focusing said broadband beam from said bottomsurface of said sample in said third light path, after transmission ofsaid broadband beam from said top surface of said sample through saidsample.